Cross-control compandor used as echo suppressors



March 4, R958 P. G. EDWARDS. ET AL CROSS-CONTROL coMPANDoR USED As ECHO suPPREssoRs Filed Feb. 24, 1954 3 sheetssneet 1 /lVl/E/VTORS A A Tom/Ev Mwah 4, i195@ P. G1. EDWARDS ET AL CROSS-CONTROL COMPANDOR USED AS ECHO SPPRESSORS Filed Feb. 24. 1954 5 Sheets-Sheet 2 llllll vous ATTORNEY /N VEN TORS United States Patent O CROSS-CONTROL COMPANDOR USED AS ECHO SUPPRESSORS Paul G. Edwards, Verona, Donald D. Robertson, Morristown, and Joseph 0. Smethurst, Bloomfield, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 24, 1954, Serial N o. 412,224

2 Claims. (Cl. 179-170.2)

This invention relates te two-way signal transmission systems, and, more particularly, te compandor circuits for reducing signal distortion and noise in such systems.

A compandor consists of a signal-controlled device called a compressor at the transmitting end of a four-Wire system or the like and a similar signal-controlled device called an expander at the receiving end ef the system.

The compresser operates to raise the level of signals of lower amplitude above the noise level of the communications medium before the transmission to the medium, while concomitantly reducing the higher amplitude signals to a level which will not overload the ampliers er the like. The expander at the receiving terminal operates in a reverse manner to restore the received signals to their original relative amplitudes, which they had at the transmitting end.

To obtain proper operation of a two-Way signal transmission system, for example, a four-wire telephone circuit, it has been found desirable in the past te employ at the terminals or intermediate points in the system, signal-controlled switching apparatus, known as eche suppressers, to disable the transmission path for one direc tion while signal transmission is taking place over the path for the opposite direction, se as to prevent reflected signal energy er echoes from being returned to the transmitting end of the system and causing a disturbance or singing.

Cross-control circuits in compandors have been disclosed heretofore in United States Patent No. 2,164,344, issued to A. C. Nerwine, and in United States Patent No. 2,328,951, issued September 7, 1943, to N. W. Bryant.

An object of the invention is to utilize such compandors to provide effective echo suppression.

Another object of the invention is to provide improved and more economical operation of four-wire telephone system by dispensing with special eche suppressor circuits through the use of compandors with cross-control from the compressor to the expander.

Another object of the invention is te utilize cempanders as eche Suppressors and facilitate break-ins by the talkers from either terminal of the system.

Another object of the invention is te make low net less circuits more stable with respect to singing tendencies through the use ef cross-controlled compandors.

The above objects are accomplished in accordance with this invention by proper modification of the cresscontrel circuits between the compressor and expander disclosed in the aforementioned Norwine and 'Bryant patents, such as te electively combine the functions of a compandor and eche suppressor in one device. y

A feature of the invention is a compandor with a unilateral cross-control circuit extending from the compresser te the expander section of the compandorY te maintain the maximum expander less or even to increase the expander loss fer the purpose of suppressing echoes. I Another feature of the invention is a unilateral cross- Qontrel circuit for compandors, whereby a distant subrifice scriber may by raising his speech volume, cause the ein pandor control to momentarily exceed the compresser' control, and thereby permit a talk spurt to pass through the expander and eiect a break.

In the aforementioned Nerwine patent, the original purpose of the bilateral cross-control circuits was te` reduce the effects of circuit net loss variations on the singing and eche characteristics of the system. This was accomplished by causing each compressor or expander to control not only its own characteristic but the characteristic of its associated expander er compressor in such a way that the round trip net loss of the echo was substantially unaffected.

In the United States patent te Bryant No. 2,328,951, issued September 7, 1943, there is disclosed a unilateral cross-control circuit from expander to compresser. This arrangement has the advantage ef improving the echo over that attainable with the bilateral cross-control of the aforementioned Nerwine patent. With the unilateral type of cross control, the increase in eche loss cannet exceed net loss plus return less. Such a method is ebviously not well suited te low net loss circuits where a large improvement is desirable, and the arrangement of the Bryant patent involves the use of an auxiliary echo suppressor in which a relay, operating from the expander, controls a separate suppressor less in the associated cem-A presser circuit input to provide a maximum added loss of 6 decibels as lixed by breaking characteristics.

In accordance with a preferred embodiment vof the inventien, cross-control is obtained by having a compressor control its associated expander, in the opposite sense to the normal expander self-control, so that the full expander less is held in the receiving branch of a feurwire circuit. When the compressor control due te speech exceeds the .expander control due te echo, theexpandor loss will be held at its maximum value to suppress any echoes. Breaking is accomplished in the same manner as used in a dilerential echo suppressor. When the local subscriber is talking the distant subscriber may, by raising his speech volume, cause the expander control te momen-y tarily exceed the compressor control and permit a talk spurt or part ef a talk spurt te pass through the expander and effect a partial or complete break.

Referring te the figures ef the drawing:

Fig. 1 shows a schematic ef a four-wire circuit with cross-controlled compandors at terminals thereof; and

Figs. 2 and 3 shew specic forms of cross-controlled compandors in accordance with the invention.

Referring te Fig. 1, the compressor 1 in the east terminal compresses the input range of speech volumes fer passage over a communications medium such as line 10, where a variety of noise and cross-talk interferences are present. The expander 2 in the receiving terminal expands the received range of compressed speech volumes te the original range.

When the circuit is idle, 28 db ef gain is'introduced by the compresser 1 or 3 and 28 db less by the expander 2 er 4. Any disturbance in the transmission medium in the absence of speech receives 28 db attenuation in the expander 2 er 4.

Loss is removed from an expander as the speech volume increases. In a well-designed compandor with proper time constants, the increased speech volume will tend pression in the expander.'

A standard hybrid coil 6 and balancing netwerk 7 indicated in Fig. 1 serve te segregate the two directions of transmission at the E, W terminals. The line 10 between the two-terminals EW, maycarrje Ythe addedimprovement in echo loss.

. guencies.

3 voice Yfrequency or carrierv link.

Y Net loss variations in a communications channel are proving the'loudness ofthe speeclnalso may be undesirable if too great because speech currents reflected from the far` end of a circuit then maybe audible as an annoying'echo. If the negative variation VisV extreme,'the circuit finally may become unstable and sing. t g j vA unilateral4 cross-control circuit 5, extendstrom a compressor 1 to its associated expandorv 4' inY theJ same Y terminal, and serves lto suppress echoes effectively enough to eliminate the need oflspecial echo vsuppression circuit. Y When the compressor cross-control due to speech ex: ceeds the expandor control due to echo, the'expandors Vvario-losser will beheld Vat its maximum value tol sup-V press'any echoes. When the local subscriber is talking,

Vthe distant subscriber may, by raising his speech volume cause the expandor control to momentarily exceed the Y fm' and it may include a radioY compressor'control to remove theexpandor loss and per-Y mit Va talk spurt or aV part thereof torlpass through the 1 expander and efect a partial Vorrcomplete break.` 'This type of cross-control will produce a different eff' fect on the operation of a four-wire toll-circuitthan has previously been encountered with `echo `Suppressors orV ompandors. Suppose, for instance, Vthat speechY is applied to the A end of the four-wire circuit andthat the net loss plus return loss at the far end Aof the circuityB Y isY low. In this case, a strongecho signal will be applied to the B end'compressor whose cross-control will insert 'additional loss Vinto its associated expander-'2m increase the effectiveV net'lossvand reduceV theV received volume to the B subscriber. For low input levels,the added net loss is zero and it approaches a'maximum of 1l Ydb at Vthe hightalker-Ylevels when theInet loss yplus return loss is zero. As a'result of'this, the far end cross# c'ontrol will produce a circuit whose' net loss is variable and afuuction of the talker volume. Limiting'the-high volume talkers also proves to be beneficial Abecause of V pand'or .circuit wherein theV control is achieved by Va charged condenser,subjected to charging by. thefcorn'-A pressorandfexpandor, respectively.4 Y Y, Y 'Inputj'voice`- signals vtlthe compressoi'jlv pass through theV germanium vario-.losser 12 thereof, and arefcom-V P'ess'ed to ,halfV the input volume range,thence. theyare 18 in the normal way, removing loss for increasing inf Y tezdis'charge .this voltage.V Conversely,"whencapaeitrV i6 is Ycharged with la negativepotential,tlie compressor reetifierBtends to discharge,thevoltage-andto make.' the desired'action inoperative. To preventthese'dischargesY from .-Qcsurrss., guard ciruirs; are illuminanteY vinto the control circuit to isolateVV the rectifiers,K B and C from capacitor 16. vkTrione*"vacuuin tubesfi' and 22, connected as cathode followers, and varistors 23 and 24Y are used -tfor this purpose. 4 Y i 'l'r'i theidle condition, with no inputto ,either` compres; sor 1 or expander v4,the"potent'iomefe'rs P1'a`ndfP2a1-fe adjusted so that 'a condition of zero'D.'C potentialcexbetween the'slide armsI of the potentiometers .and ground. Forfexafmple, as'shown theplatebatt'ery may proy'ide +130 `volts andthe cathode battery-48 volts, both with respect 'to ground. Accordingly'the voltage divider comprising thespce'path oftube `21, Ypte'nti.-- oni'e'teLPl, and resistorRa permit that there -rnaybe takenQO at the potentiometer slider any' voltage with respect to gr'oundwit'hin awide range YincludingV zero. E r'orn this'it followsv that'the D. C. Ypotential across capacitor 16 isze'r'o repftrifiers B and C.

, When input isYA applied to 'the compressor 1, rectifierjA will chargecapacitor 16 lto apositive potential. Varistor `35 23`is p' oled"so that atpos'itive voltage acrosscapacitor216A will cause' current to'flow' through it, reducing'its resist- V rectifier B to decrease lthe chargeion capacitor-16,- At' the same time, a positive potential on ,capacitor16 will cause varistor 24v to remain at its, highest resistance level and the full' voltage across Ythe capacitor will applied throughfresistor R2 between gridY and ground of tube 22.VV The cathode follower action-oftubeg22willV Vthen cause its cathode to'becomerpositive, by v2in-amount;`

Vnearly-ezp'la'ljto the potential across capacitor V'17,6 and Y i be applied in seriesV with rectifier Cto oppose the YOlagej amplified in Variplifier 15, which may be a two-stage feed- Y black amplifier 'or the like. YVMost of the amplifier output is fe'd Vthrough a compressor control amplifier 19, whence' it is rectified ina Vbridge rectifier A, having germanium Vrectifier elements.V lA condenser-resistance filter passes only the rectified syllbic envelope of they speechY fre'- VV'The positive output lof rectifier A u'sfedY to control f thefvariolosser 12'while a secondrectiiier B connected to the amplifier 19 'serves' to provide the voltage for Vthe cross-'control circuit 13. Y

Y, Y The rectified current from rectifier B is poled .to'charge capacitor 16V in arpositive direction with respect to ground 17.V

l Y Y. The 'expander 4lshown:in the upper 'right hand section has a'rectier C connected to furnish alnegative rectified Y cancun-)Which willlcliargejeapacitor' f1.6` minus with.' re- V ,stgaeetitof-ground'17.Y WhencapacitcrxrV 16zis` charged nega= tively,with respect Yto ground, therectified currentsup- Y Y fllie'd by-rectier Will-.operatezthe'zexpandor Vvario-losser acrossl capacitor 16.V flhe'eiectof this action is-,tohold the resistance of rectifier C atfa fairly khigh value and-V prevent'mitvfrom discharging thefcontrol potential acrossYY capacitor- 16 act in the same maunerjasedescribed above ,toqprevent Y thedisharge of capacitorlti. Resistor'Rl-controls the hangovertimeV to 1hold ther-expandor-lo'ss high for: echoesAV and resistor Rgis used to providegthe normal .recovery` t tinievof `:the e'xpandor. l

1, j'rl'heV cross-control k,circuit shownon Fi-g';Y 3Y 'provides` g separatef,and-independentfcontrolsforthe .echo andfeX- Y pandonhangover-times; ''he'compressorcircuit 31 lias ai separate rectiedgoutput Bfzbrought vout tooperate-.the` 'Y crosrsgeontrolgcircuittasfin Figpih4 'The output of. thisY rectifier 'is connected to :thegridgof tube 5:41 Ilocated; thevlcrplssk-,control eircuit140through1a variable resistance;`

capacitance vnetwork F32 to'iprovide .the echo hangoverl timesY requiredr yfotovarious-.ilengths' of toll, circuits.- The expando'r controlgrctiiier C," has'zits" output applie'cttot'hc' l grid;cireiu tiofiuhe42:Y Heretagaingaiseparate resistance#Y YYcapacitance networle'f is nsedftofrprovidef thetlra'n'go'verf for; a'normal a expander;

and no voltage is inserted inserics theexpandor lcontrolcircuit-,-tube21 and rectifi'enB vwill Y and connected to form a balanced bridge. The expander vario-losser 36 is connected across the balance points, which in this case are the cathode terminals of tubes 41 and 42. A potentiometer 37 is used in the common plate connection between the two tubes for adjusting the losser current to meet the input-output characteristic of the expander at its minimum input level.

When an input signal is applied to the compressor 31 the cross-control current from rectifier B will ow through the grid resistor te ground making the grid of tube 41 more positive with respect to its cathode. The cathode of tube 41 becomes more positive than the cathode of tube 42 and a voltage is impressed across the expander losser 35 to hold it in the high less condition. An additional less of about 20 db is obtained to improve the echo suppression by utilizing additional balancing impedance elements 38, 39 shunted across the expander losser 36. As shown in Fig. 3, two germanium varistors 38 and two resistors 39 are used as the balancing elements te make a lattice network which is effective only when the compressor is operated.

When input signals are applied to the expander 35, the rectitied current from its control rectifier C will ew through the grid resistor of tube 42, to ground and malte the grid more positive with respect to its cathode. The cathode of tube 42 then becomes more positive than the cathode of tube 41 and current will ow through the expander lesser, in the forward direction, te remove loss in the normal way. At the same time, this voltage is impressed across the balancing varistors in the reverse direction, to make their resistance very high so that the balancing netwerk will be etectively removed from the losser circuit. This type of cross-control circuit will add echo loss in three diierent ways: (1) the normal loss inserted inte the receiving side by the voltage impressed across the expander losser by the compressor control circuit, (2) the additional loss obtained by the balancing network in the expander losser, and (3) the normal expander loss which will be proportional to the diierential voltage existing between the cathode of tube 41, due to speech operation of the compressor, and the cathode of tube 42 by the operation of the expander on echoes.

The net result of the cross-control was found thus to insure that as the circuit input and the corresponding echoes increase, the eche loss also increases se that for the highest level talkers there is an eche less of about 60 db depending upon the return loss.

Part of this high eche loss is obtained by the impedance elements 38, 39 inserted into the expander losser circuit 35. Since higher losses would add further echo improve- Y negarse ment, a study was made to determine the optimum input, to the compressor 31 at which the expander 35 should be balanced to secure the greatest eche losses over the widest range of inputs. There appears to be an optimum input to the compressor at which the expander can be balanced te increase the echo loss over a wide range of useful inputs. Rebalancing the losser for inputs above or below this optimum value will degrade the performance at the higher input levels where the greater losses will add the most echo improvement. At high input levels the added echo loss is about 20 db and tapers el to show no improvement at the very low input levels where echoes are less bothersome.

What is claimed is:

1. In a telephone station for two oppositely directed one-way transmission paths, a volume compressor in the transmission path, a volume expander in the receiving path and a unilateral cross-control circuit for controlling the action of the expander in response to the regulation exercised on said compressor, said cross-control circuit comprising a bridge having a cathode follower amplilier in each of two arms thereof, one of said amplifiers being controlled by a voltage proportional to the output of the compressor and the other of said amplilers being centrolled by voltage proportional to the input of said expander, said bridge being balanced in the absence of an output from said compressor and a signal input te said expander, and connections from balanced peints on said bridge te said expander te control its action in an opposite sense to which it is controlled by input signals thereto in response to voltages proportional to the output of the cempandor and to override said controlling action in response to voltages proportional to inputs to said expander above a predetermined level and relative te the level of the voltage proportional te the output of said compressor.

2. In a telephone station according to claim l, a time constant circuit in the connections for supplying the voltage proportional te the output of said compresser to said control circuit for determining a predetermined hangover time for echo less and an independent time constant circuit in the connections for supplying the voltage proportional to the input te said expander to said cross-controlled circuit for providing a predetermined recovery time for said expander.

References Cited in the tile of this patent UNITED STATES PATENTS 2,164,344 Norwine July 4, 1939 

